DNA replication in mammalian cells after exposure to factors of a physical, chemical or biological nature. III. 5-Fluorodeoxyuridine induces DNA synthesis in cells not suppressed by gamma irradiation

The influence of preincubation of HeLa and Chinese hamster V79 cells with fluorodeoxyuridine (FUdR, 10(-6) M) on DNA replication and molecular weight of nascent DNA was studied after gamma-irradiation with a dose as much as 10 Gy. The 60Co-radiation inhibits DNA synthesis in both HeLa and V79 cells by 30-40 per cent. The incubation with FUdR before irradiation suppresses the inhibitory effect of irradiation on DNA synthesis. It is suggested that differences in gamma-radiation inhibition of DNA synthesis may result from the FUdR-induced changes in chromatin structure, rather than from synchronization of cell growth. This suggestion is based on the observation that the radioresistant mode of DNA synthesis occurred 18 hours following the short-term (6 hours) incubation with FUdR in cell cultures differing from each other in almost 2-fold their cell longevity.